Conventionally, Hybrid Auto Repeat reQuest (HARQ) technology is known as a method of increasing transmission efficiency and transmission quality in digital signal transmission. HARQ is a transmission method that combines error-correction and retransmission-control. Specifically, it transmits a packet of transmit data that has been error-corrected, and, if that packet is not successfully received, transmits another packet created based on identical transmit data; the original transmit data is then successfully extracted from these multiple packets on the receiving side.
Representative examples of HARQ methods are Incremental Redundancy (IR) and Chase Combining, which will be explained using the drawings.
{IR Method}
FIG. 9 is a block diagram of a transmitting apparatus 30 that performs digital transmission using IR, and FIG. 10 is a block diagram of a similar receiving apparatus 40.
In FIG. 9, an information bit for transmission is input to an encoder 301 which corrects errors and outputs a codeword including an information bit and a parity bit. Of the output codeword, a first transmit block including the entire information bit and a bit including the front half of the parity bit, and a subsequent transmit block including the remaining parity bit, are accumulated in a buffer 302, and these blocks are sequentially input to a modulator 303. The modulator 303 modulates the input blocks to create corresponding modulation symbols, and a transmitter (not shown) transmits these modulation symbols. An information bit is thus transmitted by first transmitting a first transmit block, and then a subsequent transmit block.
In FIG. 10, a demodulator 401 demodulates an initially-received first receive block (corresponding to the first transmit block), and outputs it to a block-combining circuit 402. Since the subsequent transmit block has not yet been transmitted, the block-combining circuit 402 processes the untransmitted parity bit as a punctured (invalid) bit, and outputs the input first receive block unaltered to a decoder 403. The decoder 403 decodes the input block and extracts the information bit; since it does not used part of the parity bit in the entire original codeword in FIG. 1, the decoder 403 decodes it at a higher coding rate than when the entire codeword is used.
When the first receive block cannot be successfully decoded during this decoding process, the receiving apparatus side reports this fact to the transmitting apparatus side, and the receiving apparatus 30 that receives this report transmits a new subsequent transmit block.
While the subsequent receive block (corresponding to the subsequent transmit block) is demodulated and input to the block-combining circuit 402 in the same manner as above, this time, the first and subsequent receive blocks are sent to the decoder 403 after being combined in series. The decoder 403 decodes the entire combined block, and extracts the information bit. The coding rate at this time is lower than the coding rate when only the first receive block was decoded. Therefore, the error-correction ability of this decoding is enhanced, and decoding can be achieved more reliably than the first decoding.
{Chase Combining}
FIG. 11 is a block diagram of a transmitting apparatus 31 that performs digital transmission using chase combining, and FIG. 12 is a block diagram of a similar receiving apparatus 41.
In FIG. 11, in chase combining, a codeword is duplicated in a memory 304 and sent to a modulator 303. It is separated into a block for each duplicated unit (i.e. each original codeword), and these are sequentially output as a first transmit block and a subsequent transmit block. That is, in chase combining, the same data is transmitted in first and subsequent transmissions. The memory 304 can accomplish the same operation if it is provided in a rear stage of the modulator 303. That is, in chase combining, the same data is transmitted in first and subsequent transmissions. The memory 304 can accomplish the same operation if it is provided in a rear stage of the modulator 303.
In FIG. 12, a process identical to IR is performed to the first receive block. When a subsequent transmit block is transmitted after decoding was unsuccessful with only the first receive block, the block-combining circuit 402 combines the first and subsequent receive blocks at the same phase, and outputs this to the decoder 403. Since the power per bit of the input to the decoder 403 is greater than in the first time, decoding is more likely to be successful than the first time (unlike IR method, the coding rate does not change between first and subsequent times.)
Transmission can be performed by combining the above IR and chase combining methods, instead of using them separately. FIG. 13 is a block diagram of a transmitting apparatus 32 that performs transmission using such a combination. In FIG. 13, a codeword is duplicated in the memory 304, and then separated into four blocks (first transmit block to fourth transmit block) which are sent to the modulator 303. The first and third blocks include the same information bit and part of the parity bit as the first transmit block in IR method, while the second and fourth blocks include the same remaining parity bit as the subsequent transmit block in IR method.
On the receiving side (a block diagram of the corresponding receiving apparatus is not shown), a process performed when the first and second receive blocks are received is the same as in the IR method described above, whereas, when the third and fourth blocks are received, these newly received blocks are combined at the same phase with the blocks that are already received, and decoded, in the same manner as in chase combining described above.
When applying the various HARQ techniques described above in a multicarrier transmission system using a great number of frequency-divided carriers, while transmission parameters are usually made the same for every transmit block, Non-Patent Literature 1 introduces a method of partially increasing throughput in which, when using IR, preferably, Walsh code spread multiplexing transmission is performed only when transmitting a first transmit block including only an information bit, and code spread multiplexing is not performed when transmitting a parity bit
{Non-Patent Document 1} Takaoka, ‘Throughput Characteristics of MC-CDMA HARQ using Adaptive Variable Diffusion Rate’, Shingaku Giho, Institute of Electronics, Information and Communication Engineers (IEICE), July 2005, RCS 2005-43, pp. 19-24.
In the digital signal transmission method using IR described above, while frequency diversity is achieved by transmitting many parity bits in second and later transmissions, a downside is that deterioration factors such as inter-code interference due to de-spreading of the spread multiplex signal become conspicuous. In chase combining, since the receive S/N increases due to block combining in second and later transmissions, this method does not offer the advantage of relatively reducing the effects of inter-code interference. Thus, since conventional HARQ methods do not always optimize the transmission parameters, throughput becomes problematic, and high-quality transmission cannot be realized.